This invention relates to apparatus for the control of induction motors and particularly to means for controlling the speed and torque of such motors in a way which permits rapid response to control signals when such motors are employed in servo systems.
In a servo system where the motor torque and speed characteristics are varied in accordance with a program or schedule, it is desirable to have a motor which responds quickly to programmed changes in its condition. It has been customary in the past to use d.c. motors for such applications, but d.c. motors have the major disadvantage of requiring brushes and a commutator assembly which adds to the complexity of the motor system and the amount of necessary maintenance. It is, therefore, desirable to employ a.c. motors, and particularly induction motors for this purpose, so that the simpler construction of the a.c. motor can be used to advantage. However, when an induction motor is operated at less than its rated speed, a relatively high slip value results in increased heating within the motor, an undesirable condition. Such heating not only renders motor operation less efficient, but may also damage the motor.
In an effort to employ an a.c. motor in a servo loop, it has been proposed to supply a variable frequency to the motor, so that the motor can operate over a variety of speeds at a substantially constant slip. It has also been proposed to vary the input voltage to the motor so that the output speed of the motor is a function primarily of the voltage input, and to combine both methods of control so that the amplitude of the driving voltage, and its frequency, are controlled together by some fixed relation. However, the means which have been developed in the past for doing this have not been entirely successful. The responses of the motor to commands for changing values of speed and torque have been relatively slow, and the apparatus employed has not permitted a high degree of precision and accuracy in controlling the motor, nor any flexibility in the relation between frequency and voltage control.
One problem has been that with low frequency operation, there is a relatively long time between successive state changes of the windings which generate the stator field. In a three phase motor, for example, there are only six state changes per cycle, and the time interval between successive state changes, viz. 1/6 cycle of the rotor, becomes quite long at low rotor frequencies. Moreover, the flux which traverses the air gap between stator and rotor can be changed in value only relatively slowly, because of the high inductance of the rotor. These and other problems combine to suggest that the use of an induction motor is not feasible when precise control is required at low rotor speeds.
It is, therefore, a principal object of the present invention to provide a mechanism whereby precise control of the speed and torque of an induction motor is attained, even at very low speed conditions.
A further object of the invention is to provide for a flexible relation in controlling the frequency and voltage of the a.c. power applied to the motor.
Another object of the present invention is to provide a mechanism for controlling an induction motor without causing a high degree of heating within the motor when operated at low speeds.
A further object of the present invention is to provide mechanism whereby control of motor speed and torque are controlled by periodically recurring control signals which have a relatively high frequency in relation to the speed of the motor.
Another object of the invention is to provide digital apparatus for accepting periodic digital control signals from a numerical control apparatus and employing such signals to effect precise control of an induction motor.
A futher object of the present invention is to provide apparatus and a method for generating periodic digital control signals in accordance with a specified algorithm relating to the speed of the motor and servo following error to the frequency and amplitude of the driving voltage applied thereto.
Another object of the present invention is to provide apparatus and a method for periodically calculating digital values for the frequency and amplitude of the driving voltage as a function of motor speed and following error, such function being derived empirically for the motor with which the apparatus is used.
A further object of the present invention is to provide apparatus and a method for directly controlling the position of the resultant magnetic flux generated in the stator of the motor as a function of the motor speed and servo following error.
Another object of the present invention is to provide apparatus for disconnecting the motor from its driving voltage in response to a condition of excessive motor current.
A further object of the present invention is to provide apparatus and a method for modifying the number of poles in a multiphase a.c. motor, in response to a calculation involving motor speed and servo following error.
Another object of the present invention is to provide apparatus and a method for calculating the frequency of the driving voltage as a function of motor speed and servo following error, and selecting one of a series of discrete frequency values in response thereto, and applying a hysteresis quantity in said calculation for establishing one criterion for selecting one frequency, and a different criterion for selecting another frequency subsequent to selecting said one frequency.